1. Field of the Invention
This invention relates to a method for coded, sequential, noncoherent, redundant, optical data storage wherein a number of one-dimensional holograms are reproduced upon a light-sensitive tape in a timed sequence with the help of a pulse amplitude-modulated light beam, and a correction exposure of the light-sensitive tape, which is separate from the signal effected exposure with respect to space and time is effected in such a way that the entire exposure resulting from the signal exposure and the correction exposure is constant during each recordation, and more particularly to a method of the above type wherein one of the beams is noise intensity modulated with respect to time to provide a contrast of the individual holograms which statistically varies.
2. Description of the Prior Art
In sequential optical data storage, the process was heretofore such that a time variable signal was modulated upon a laser beam by means of pulse amplitude-modulation, and this intensity modulated laser beam was caused to interfere with a coherent timely constant reference wave upon a light-sensitive storage medium. The resulting interference figure is then recorded as a one-dimensional hologram upon a moving storage tape. Each sensing pulse is associated with a hologram whose contrast is a measure of the level of the corresponding pulse.
In a case such as set forth above, each sensing pulse will produce a new hologram. Therefore, the mechanical stability conditions which are common in holography must be maintained by the recording system during the recording interval. For this reason, and due to their coherent properties, only lasers could be considered as practical light sources.
In order to avoid these drawbacks, it has been suggested for sequential, non-coherent, redundant, optical data storage to arrange a one-dimensional hologram ahead of the light-sensitive band, which hologram is recorded upon the light-sensitive band by a pulse amplitude-modulated, non-coherent light beam. In order to further prevent a movement of the operational point in the individual exposures along the characteristic of the recording medium, it was also suggested to provide a correction exposure of the light-sensitive band, the correction exposure to be separate from the signal exposure with respect to space and time in such a way that the entire exposure resulting from the signal exposure and the correction exposure is constant during each recording operation. In this connection, one may refer to German patent application No. P 21 48 649.7-53 or to the corresponding U.S. patent application Ser. No. 290,796, filed Sept. 21, 1972.